Design Of Simplified Active NPC (ANPC) Inverters For Fault… If we assume that the capacitors and devices can survive in this condition, the inverter output currents will become unbalanced.Fig.2 (a), then the phase output is connected to Neutral-point (NP) of dc- link instead of positive dc bus. (b)When Da 1 open fault occurs at “+” state and Ia< 0, as shown. (c), Sa 2 open failure occurs at “0U2/0U1” state when Ia> 0, then the phase output is connected to negative dc bus rather than NP of dc-link. (d) Shows that Sa 5 open failure occurs at “0U2/0U1” state when Ia< 0, then the phase output is connected to positive dc bus instead of NP of dc-link
Fig.(d) open circuit condition (Sa2 open fail) . Due to the incorrect output voltage, the output current will become unsymmetrical and the NP of dc-link will be unbalanced, the condition is even worse since the phase current Ia becomes discontinuous due to the cutoff of conduction path
Fig.3 Modes of operation – (a) short circuit condition (Sa1/Da1 short fail) then the induced voltage on load inductor and loop inductor may cause overvoltage on the inverter and cause damage. For other device failure cases, the circuit can be analyzed in the similar way. Device short failure can cause even more serious problems compared to open failure. The reason is that under short-failure Condition,
Fig (b) short circuit condition (Sa2/Da2 short fail,-ve) the dc-link capacitors may be discharged through a short current conduction path directly, and some devices may break down due to over current.Fig.3 shows the current flow path under short failure of Sa 1 /Da1 , Sa 2 /Da 2 , and Sa 5 /Da 5 ,respectively. When Sa 1 /Da 1 short failure occurs, if the switching state commutates to “0U1/0U2/−,” Moreover, because the voltage of one dc-link capacitor will drop to zero quickly, other devices may have to withstand the full dc bus voltage and break down due to overvoltage
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